Noncorrosive nitrogen tetroxide containing p2o5



States U.S. Cl. 149-74 2 Claims ABSTRACT OF THE DISCLOSURE A method forthe facile control of the corrosive effects produced on metal surfacesby water-bearing nitrogen tetroxide. The method involves addingsufficient amounts of phosphorus pentoxide to the water-bearing nitrogentetroxide as a corrosion inhibiting additive. The water content of thenitrogen tetroxide preferentially reacts with the phosphorous pentoxideinhibitor to form phosphoric acid instead of reacting with the nitrogentetroxide to form nitric acid. This preference results in minimizing thequantity of corrosive nitric acid formed and allows the resultingphosphoric acid to react with the surface of the tank to form aprotective metal phosphate film.

BACKGROUND OF THE INVENTION This invention relates to a method forinhibiting the corrosion of metal surfaces by nitrogen tetroxide. In amore specific manner, this invention concerns itself with nitrogentetroxide rocket fuel oxidizers containing small amounts of water and toa method for protecting metals from the corrosive effect of suchoxidizers through the utilization of a phosphorus pentoxide corrosioninhibiting additive.

The recent advent of nitrogen tetroxide as an oxidizer for liquidpropellant rocket fuels has created a materials handling problem ofmajor proportions. Its storage and handling has been attended bycorrosion problems and metal failure especially at the welds of aluminumstorage containers. Corrosion failure is caused by the presence of waterin the nitrogen tetroxide oxidizer and is enhanced by the affinity ofthe oxidizer for atmospheric moisture.

No corrosion problem has been encountered when using anhydrous nitrogentetroxide. This is borne out by the fact that tests have shown anhydrousnitrogen tetroxide is practically noncorrosive and that when watercontent is held within specified limits, ordinary chemical-corrosionfailures do not occur. Accordingly, military procurement specificationsfor water in nitrogen tetroxide limits its concentration to 0.1 weightpercent of the material as procured, and under field conditions, waterconcentration is not allowed to exceed 0.2 weight percent water.

In practice, however, experience has shown that contamination ofnitrogen tetroxide by water is diflicult to avoid because of itsinherent hygroscopic nature as well as the circumstance that it isfrequently stored at temperatures below ambient, thus producing localcondensation of moisture in the interior of storage tanks and transferlines. Prior to this invention, no known methods were available for thefacile control of corrosion by wet nitrogen tetroxide. Consequently,corrosion failure of storage tanks with the attendant exposure ofpersonnel to the highly toxic effects from this material constituted anever present danger whenever the oxidizer was utilizedin aerospaceoperations.

One of the primary drawbacks found to exist during attempts to overcomethe corrosive elfect of wet nitrogen tetroxide was the low solubility innitrogen tetroxide of inhibitors commonly used in other corrosiveliquids. Also,

atent it had been observed that inadequate amounts of socalledinhibitors may actually promote corrosion. Many inorganic inhibitorssuch as chromates, arsenates and iodates were found to be ineffective innitrogen tetroxide because of their low solubility in this nonpolarmaterial. Organic inhibitors, such as amines, were, in general, found tobe incompatible with nitrogen tetroxide. The discovery ofa goodcorrosion inhibitor for nitrogen tetroxide therefore, became a problemof the highest practical interest.

With this invention, however, the problems encountered in using moistnitrogen tetroxide as an oxidizer for rocket fuels have been abrogatedto a great degree through the discovery that phosphorus pentoxidefunctions as an efiicient and effective corrosion inhibiting additivefor water-bearing nitrogen tetroxide. While nearly insoluble inanhydrous nitrogen tetroxide, phosphorus pentoxide will dissolve to formphosphoric acid to the extent that water is present, thus restoring themoist nitrogen tetroxide to its noncorrosive condition.

SUMMARY OF THE INVENTION In accordance with the present invention, ithas been found that the corrosive effect produced on metal surfaces bymoist nitrogen tetroxide can be easily controlled in an effective mannerby using phosphorus pentoxide as a corrosion inhibiting additive. Theinhibitor reacts with the water present in the nitrogen tetroxide toform phosphoric acid thus causing the water containing nitrogentetroxide to revert to its noncorrosive anhydrous state. Consequently,the problem of metal surface corrosion attendant with the use ofnitrogen tetroxide as a liquid propellant oxidizer has been largelyovercome.

Accordingly, the primary object of this invention is to provide a methodwhereby the corrosion of metal surfaces by wet nitrogen tetroxide issubstantially eliminated.

Another object of this invention is to provide a corrosion inhibitingadditive for nitrogen tetroxide.

Still another object of this invention is to provide a corrosioninhibited nitrogen tetroxide oxidizer for rocket fuels which is adaptedto adequately protect the metal surfaces of storage containers from thecorrosive effects of a water-nitrogen tetroxide system.

A further object of this invention is to provide a method for inhibitingthe corrosion of metal surfaces in a relatively simple, yet effectivemanner.

These and still further objects and advantages of this invention willbecome readily apparent from the description of the invention whichfollows when taken in conjunction with the accompanying drawing.

DESCRIPTION OF THE DRAWING In the drawing:

The figure presents a graphical illustration pointing out theeffectiveness of the corrosion inhibiting additive of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Pursuant to the objects of thisinvention, the present method involves the addition of phosphoruspentoxide to moist nitrogen tetroxide. The phosphorus pentoxidefunctions as a corrosion inhibiting additive thereby permitting thefacile utilization of nitrogen tetroxide as a rocket fuel oxdizer. Theinhibitor may be added to the oxidizer supply tank of a bipropellantfuel system and the fuel may be selected from any of thoseconventionally used such as monomethylhydrazine (MMH) ora blend ofhydrazine and unsymmetrical dimethylhydrazine (UDMH). The manner inwhich the inhibitor is added to the nitrogen tetroxide oxidizer is notcritical and any conventional filling means can be employed. Noparticular equipment or safeguards are essential.

The amount of inhibitor needed to control the corrosive nature of moistnitrogen tetroxide depends on the amount of water present in thetetroxide since the phosphorus pentoxide will react to form phosphoricacid to the extent that water is present. More specifically, the presentmilitary procurement specifications permit the presence of not more than0.1 weight percent water in nitrogen tetroxide oxidizers. As aconsequence, one pound of phopshorus pentoxide is required per 32gallons of nitrogen tetroxide to prevent corrosion with this percentileof water.

In order to evaluate the effectiveness of the inhibitor, a resistancetesting method Was utilized. The changing resistance of a magnesiumribbon in a corrosive medium, N plus 0.5 weight percent H 0 and variousinhibitors, was measured as a function of time. Three typical curves:(1) NH PF (2) P 0 and H 0 with inhibitor, and (3) P 0 and H 0 withoutinhibitor, are illustrated in the figure of the drawing. They have thegeneral form:

where a, the slope of the curve, is a measure of the corrosion rate. Thesmaller the value of a, the greater is the corrosion protection. Thevalues of the inhibitors tested are given in Table I.

Use of this slope in devising a figure of merit for rating inhibitorsrequires a second factor, the intercept of the curve on the Y-axis. Thisintercept should have a low value if corrosion inhibition is effective.Compensation is made for variation in the initial resistance of the teststrips by an equivalent figure, which is obtained graphically bydetermination of the time at which the resistance of the test strip hasincreased 2%. The final figure of merit is the ratio of the quotient ofthis time divided by the slope of the curve and the same quotient forwater without an inhibitor:

a log x=log y where FM figure of merit; T =time for 2% resistanceincrease, inhibitor added; q=slope of corrosion rate curve from Table I;T =time for 2% resistance increase, no inhibitor; and a 0.56, slope ofcorrosion rate curve for water.

It is apparent from Table II that P 0 has a real value in protectingmagnesium from corrosion by water-bearing N 0 P 0 is a powerfuldesiccant and is more likely to act by forming phosphoric acid with thewater in preference to the formation of nitric acid. This preferenceresults in minimizing the quantity of corrosive HNO formed. Theresulting H PO reacts with the surface of the metal to form a protectivemetal phosphate film.

In the figure of the drawing, the curves plotted for corrosion ofmagnesium with no inhibitor and with P 0 inhibitor have a low initialslope, as indicated by the dashed portion of the curve. This low initialslope was noted in most tests and may represent a period of slow removalof an oxide coating from the magnesium.

TABLE II Figure of merit for inhibitors with magnesium Inhibitor: Figureof merit From the foregoing description it will be apparent that thisinvention provides a method which is most effective in controlling thedeterioration of metal surfaces from the corrosive eifect of moistnitrogen tetroxide. The utilization of the phosphorus pentoxideinhibitor of this invention has unexpectedly facilitated the use ofnitrogen tetroxide as an oxidizer for rocket fuels and has enabled thoseskilled in the field to take practical advantage of the desirableoxidizing properties of nitrogen tetroxide.

The invention has been described with particular reference to a specificembodiment thereof. It is to be understood, however, that thedescription of the present invention is for the purpose of illustrationonly and is not intended to limit the invention in any way, the scope ofwhich is defined by the appended claims.

What is claimed is:

1. A method for inhibiting the corrosive action produced on metalsurfaces by water-bearing nitrogen tetroxide which comprises addingphosphorus pentoxide to said nitrogen tetroxide in an amount sufiicientto effect a preferential reaction between said phosphorus pentoxide andsaid water to the extent that water is present in the water-bearingnitrogen tetroxide.

2. A method in accordance with claim 1 wherein about one pound ofphosphorus pentoxide is added to about thirty-two gallons of a nitrogentetroxide solution bearing 0.1 weight percent water.

References Cited UNITED STATES PATENTS 7/1946 Lawson 149-74 XR 5/1968Croomes 60214 US. Cl. X.R.

